I've just read this entire thread. Firstly, can I just say that there is some lovely, thoughtful, balanced advice from people who are in relationships with widowers. I'm a widow who is dating a widower, but I've said on here before that our circumstances differ as my marriage wasn't entirely happy, whereas his was. The reason for saying that is the I hope the OP has found all the comments from those in a similar situation to herself useful, as it really is a very 'particular' experience, being with someone who had a happy marriage torn from them in that way.
OP, having said all that, I'm actually not convinced that the 'widower factor' is the main issue here. You sound incredibly sorted, sensible and a great person. You are protecting yourself financially while also loving your DP and his family which is great. He also sounds like a good man and clearly managed (if that is the right word) as a lone parent for many years.
I think it is just about views on the importance of marriage once you get older, and this could apply to anyone,widowed or not. What I mean is, for many people, they will marry in their 20s or 30s, and see this marriage as a framework, on which other things will be built eg children, house purchases, careers etc. By the time we are in our 40s/50s, many of those things are in place, and, if someone is not of the view that marriage is an 'end in itself' then it might not matter so much.
I was glad to read your comments about not planning to end things, but needing to think about it all. It would be very easy to have a lovely relationship brought into doubt by taking some comments to heart. Also, it is actually still quite early days in terms of your relationship, so I guess you never know.